For reference, the recipe at:
The recipe at
https://lists.debian.org/debian-alpha/2019/11/msg00033.html
for adding firmware to the install disk did indeed work.
I booted with the modified install disk in the SCSI CD drive and the original in a second IDE CDROM drive that runs on a PCI card. While the installer could not see the SCSI CDROM drive with the install disk in it once control was handed over from the initial boot (step where it looks for install media initially failed), it could see the IDE CDROM and then switched to that for the rest of the install.
Worked well.